Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, e.g., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. The fog sites on silver halide crystals can originate during emulsion manufacture or during aging of the photographic element. During the aging process, fog can become exaggerated by extremes in temperature and humidity. Fog can be defined as developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "D-min", the density obtained in the unexposed portions of the emulsion. Developed density, as normally measured, includes both that produced by fog and that produced by exposure to light.
An especially critical time in fog formation is during the sensitivity enhancing process. The sensitivity enhancing process constitutes the period from final washing of the precipitated silver halide emulsion to the point where the thermal treatment during chemical/spectral sensitization is completed. At the end of the sensitivity enhancing process, the desired sensitometric properties of, for example, speed, contrast, and reciprocity have been achieved.
Many methods have been employed to minimize the appearance of fog. Mercury-containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to combat fog. Thiosulfonate and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed. Disulfides, such as those described in U.S. Pat. Nos. 3,397,986; 5,219,721; and 5,418,127 have also been found effective in limiting fog, especially when used during the sensitizing process of silver halide emulsion manufacture.
Unfortunately, such fog-combating compounds are not without drawbacks. Many compounds which are useful as antifoggants also reduce speed. Mercury-containing compounds, while being very effective antifoggants are environmentally undesirable.
In this invention it has been discovered that a specific group of isothiazolin-ones is particularly useful in controlling fog when added before or during chemical sensitization. Isothiazolin-ones are known as useful biocides for silver halide photographic elements as described in Research Disclosure 37026, February 1995; in U.S. Pat. Nos. 4,224,403 and 4,490,462; and in JP 09-329862 and JP 10-011739. JP 09-133977 describes one class of isothiazolin-ones as reducing fog when added during precipitation of a silver halide emulsion. However, nowhere has it been recognized or described that a specific group of isothiazolin-one compounds confers significant advantageous sensitometric benefits when used during the sensitivity enhancing process of emulsion manufacture.